[Thunderbolt Q&A, Help & Troubleshooting] marcogiudice:
ok i found half my answer... this for other acquiring minds like me

Here's some (hopefully useful) information about the bootloader:

What's HBOOT?

It's immediately loaded when the phone is switched on. It's mostly responsible for checking and initialising the hardware and starting the phone's software, for flashing official software releases (RUUs via Fastboot), as well as a few other things. In some ways it's comparable to the BIOS on a PC.

How do I root HBOOT?

You can't, it makes no sense. It's a bootloader, not Android. It's like trying to play a DVD on your kettle.

What does upgrading HBOOT do?

It may add extra hardware support for new phones (e.g. SLCD displays) and may fix a few bugs. HTC doesn't release information about it.

What does downgrading HBOOT do?

The opposite. So, if you flash an SLCD phone with an old version of HBOOT that doesn't have the support, the screen won't work.

But rooting depends on HBOOT version?

It doesn't.

Why does upgrading HBOOT remove root?

It doesn't. When you flash an official update your ROM is replaced with an official one, which is why root is lost.

How do I change HBOOT version?

By installing an official RUU/update. There's no other way - the bootloader is protected by the phone's hardware security because a corrupt bootloader means a bricked phone with no way to recover. RUUs contain a version of the bootloader, which is automatically flashed.

Why do I need to downgrade HBOOT to root 0.92?

You don't. You had to downgrade Android because unrevoked3 couldn't root froyo. The new release can, so it works.

Why all the talk about HBOOT then?

Because it is confused with software release, which is the important part. An RUU won't work if the installed software is a higher version (e.g. 1.20.x won't work if you have 1.21.x), and people have mistaken that check for HBOOT version.

What about the xda 0.92 to 0.80 downgrade? It runs on froyo.

It's an engineering/test release from HTC that was leaked. It fakes its software version to 2.09.405.8 so it will run on froyo and downgrade the ROM, so long as you're not on a higher release than that. The process has nothing to do with HBOOT, but it does downgrade the bootloader to 0.80 as a side effect - and results in a dead screen on SLCD phones. It's generally a bad idea to use this hack.

Why would any of this matter?

Because instructions based only on HBOOT version are not reliable. They may not work, or may cause your phone to stop working.

OK NOW FOR FASTBOOT........

Fastboot

Fastboot is protocol used to update the flash filesystem in Android devices from a host over USB. It allows flashing of unsigned partition images. It is disabled in the production G1 devices since USB support is disabled in the bootloader[1]. This can be changed if you get root on the device. See also [2]

Fastboot prerequisites

To be able to use fastboot, you need to be running either an ADP1 or the Engineering Bootloader on your G1.

If you are developing on a linux host, you may need to tweak UDEV to recognize the phone in fastboot mode. You may already have done this to get the phone recognized in Eclipse, but you need to do it again for fastboot, since fastboot uses a different device ID: 0bb4:0c01. The warning sign is that fastboot says "< waiting for device >" forever, even when you have the "FASTBOOT" message on the phone; if this happens, try adding the device ID 0bb4:0c01 to your UDEV configuration for Android.

For Opensuse 11.1, the following config file, in /etc/udev/rules.d/11-android.rules, seems to work:

SUBSYSTEMS=="usb", ATTRS{idVendor}=="0bb4", ATTRS{idProduct}=="0c01", MODE="0666", OWNER="mylogname"

SUBSYSTEMS=="usb", ATTRS{idVendor}=="0bb4", ATTRS{idProduct}=="0c02", MODE="0666", OWNER="mylogname"

NOTE: change "mylogname" to your Linux login ID. Also note that the product ID bit is optional (probably easier to leave it out), and you need to change "0bb4" to the vendor ID for your phone if it isn't made by HTC (whose vendor ID is 0bb4). You can find the vendor id using 'lsusb'. An alternative to this is just to run fastboot as root, using sudo.

Starting Fastboot

On the G1: To start fastboot, first get to the bootloader: turn off your phone and hold down the "camera" button while powering it back on. You will see a white screen with skateboarding bugdroids. If you just see the normal multicolored bootloader screen, you need to get the device side of fastboot working as noted above. Plug your phone into the usb and if you see "serial0" hit the "back" key. You should now see "fastboot" on the screen.

On the Samsung Galaxy: power down the phone, then long-press the two Call buttons. Beware that fastboot mode will continue to reboot the phone after ~30 seconds of inactivity.

Fastboot Binary

To make use of fastboot, you also need the fastboot program compiled for your host computer. If you have already built Android, you will have this file already. Its location should be:

<android source location>/out/host/{os}_{arch}/bin/fastboot

If you do not have fastboot built, you can either compile the Android source or try DarkriftX's precompiled fastboot (compiled on kubuntu x86)

Fastboot Commands

Here are the commands you can run on your host after fastboot has been started on a device connected via USB. This is a paste directly from the fastboot binary's "--help" command:

usage: fastboot [ <option> ] <command>

commands:

update <filename> reflash device from update.zip

flashall 'flash boot' + 'flash system'

flash <partition> [ <filename> ] write a file to a flash partition

erase <partition> erase a flash partition

getvar <variable> display a bootloader variable

boot <kernel> [ <ramdisk> ] download and boot kernel

flash:raw boot <kernel> [ <ramdisk> ] create bootimage and flash it

devices list all connected devices

reboot reboot device normally

reboot-bootloader reboot device into bootloader

options:

-w erase userdata and cache

-s <serial number> specify device serial number

-p <product> specify product name

-c <cmdline> override kernel commandline

Forcing fastboot to load on every boot

Developers who are creating new images to try out very often can remove their boot and recovery images which will force the phone to enter bootloader mode every time you boot. To fix this, you would reflash the boot and recovery images back allowing the phone to boot normally.

$ fastboot erase boot

$ fastboot erase recovery

NOW,

SO WHY DO WE CHANGE RUU.'S TO PG05IMG??????????

AND THE COUPLE OF OTHER OPTIONS DO IN BOOTLOADER?

THANK U!